Light emitting diode (LED) technology enables high efficiency white, red, green and blue light sources with lifetimes of approximately 100,000 hours. White emitting LEDs are now in widespread use for liquid crystal display (LCD) backlighting and are entering general lighting markets. Red, green and blue LEDs are widely used in large video displays where each LED is directly viewed and electrically controlled. LED displays are bright and can be large however the resolution measured in total pixel count depends on the number of LEDs. For example a full colour high resolution screen having 1080 rows and 1920 columns requires 6,220,800 LEDs. In many applications where high resolution colour displays are used, this approach is costly.
In conventional colour LCDs, the standard approach has been to use a bright white light source as a backlight, and to employ an addressable colour LC (liquid crystal) light modulator that includes a colour filter array to control the colour and brightness of each pixel. This technology has become the leading solution for television displays. LCDs are also used for smaller computer and handheld displays. White LEDs have been adopted as a preferred backlight technology for LC displays of all sizes. This LC technology is limited by the glass size of the LC modulator. Both plasma and liquid crystal displays rely on glass sheets and are not readily available in sizes above approximately 100 inches. Making much larger units is a challenge due to glass processing and transportation issues.
For larger displays such as video walls, the use of tiled LC or plasma displays is popular. Videowall displays are typically a set of flat panel displays placed as closely as possible to each other. The desired image may be spread across the set of displays, however there are gaps between the active areas of these displays that generally detract from the viewing experience. These gaps are formed by the bezels surrounding each display and are often referred to as bezel gaps. Bezel gaps as small as a few millimeters are achievable with premium LC and plasma display videowalls and are often a few centimeters in lower price videowalls.
A significant disadvantage of the aforementioned LED displays as well as LCD-based and plasma based videowalls is the inability to economically produce large size, high resolution displays having no visible bezel gaps. Currently, seamless tiling between display units can be achieved using projection display systems. A technique known as edge blending allows multiple projectors to overlap their images on a single screen. Such systems are not flat panels and are not suitable for certain applications such as wall mounted displays where a thin form factor is required.
An array of projectors comprising LC light modulators backlit by LEDs has been identified as an approach to making displays of arbitrary sizes. US Patent Application US2008/0284677A1 (WHITEHEAD et al) publication date Nov. 20, 2008 teaches the use of an array of discrete modules, each comprising a projector having a processor and a light modulator, projecting on a screen to form an image. Light from a given module may overlap with light from adjacent modules at the screen. The overall display size is dependent on the number of modules.
In U.S. Provisional Application No. 61/240,412 “LIGHT EMITTING DIODE ILLUMINATED DISPLAY” filed on Sep. 8, 2009, the use of multiple LC light modulators, each containing arrays of LEDs is disclosed. Specifically the elimination of bezel-gaps using edge blending has been disclosed. This concept was further developed in PCT application No PCT/CA2010/001407 “LIGHT EMITTING DIODE ILLUMINATED DISPLAY” filed Sep. 8, 2010. In order to eliminate gaps between LC modulators the LEDs near the edges of each modulator may be tilted or splayed out to project light onto the screen located a short distance in front of the LC modulators such that all areas of the screen are illuminated. This approach is shown to form a practical and thin display since only a short distance is required between the screen and the LC modulators.
The use of arrays of LEDs behind each LC modulator reduces the number of LC modulators required and decreases system complexity and cost. One problem with this approach occurs because rear projection screens are sensitive to the direction from which light strikes the screen. The screen image, as viewed by the viewer, will not maintain appropriate brightness levels as the viewer views the screen from a range of viewing angles. This causes image artifacts that the viewer notices at boundaries between adjacent LC light modulators. Other problems with this approach are high power consumption due to light loss in the light modulators and screen, a lack of screen contrast due to both ambient light reflecting off the screen, contrast limitations of the light modulators and the degradation of light modulators due to the high brightness LED arrays behind the LC light modulators.
It would therefore be advantageous to enable the use of multiple LC light modulators, each containing arrays of LEDs, to project an image onto a screen without visible image artifacts and without visible gaps between LC light modulators. Furthermore it would be an advantage to achieve a reduction in power consumption, an increase in screen contrast and a decrease in the degradation of the light modulators.